Highly selective radiofrequency (RF) filters are used in a variety of applications to avoid interference between adjacent bands, including TV broadcasting, GPS systems, mobile phone systems, and wireless local area networks. For example, mobile phones need RF filters to protect the sensitive receive path from interference by transmit signals from other users and noise from various RF sources. Bandpass RF filters operate in the 100 MHz to 10 GHz frequency range and preferably have extremely low losses and high quality factors (Q). In particular, RF communication and radar systems rely on high-Q resonator filters at increasingly high (>1 GHz) frequencies. Further, filters need to have a specified impedance level at the input and output ports. Many systems work at 50Ω impedance level, because this is a reasonable compromise between the size and performance of components and the currents and voltages occurring inside the system. See R. Aigner, “MEMS in RF filter applications: thin-film bulk acoustic wave technology,” Sensors Update 12(1), 175 (2003).
Previous research has demonstrated that arrays of low-impedance, temperature-compensated piezoelectric lateral microresonators spanning the very high (VHF: 30-300 MHz) and ultra high (UHF: 300-3000 MHz) frequency range can be realized in a compact size on a single chip. See R. H. Olsson III et al., “Post-CMOS compatible aluminum nitride MEMS filters and resonant sensors,” IEEE Freq. Cntl. Sym., 412 (2007); R. H. Olsson III et al, “VHF and UHF mechanically coupled aluminum nitride MEMS filters,” IEEE Freq. Cntl. Sym., 634 (2008); G. Piazza et al., “Single-chip multiple frequency AIN MEMS filters based on contour-mode piezoelectric resonators,” J. of Microelectromech. Syst., 16(2), 319 (2007); and W. Pan et al., “A low-loss 1.8 GHz monolithic thin-film piezoelectric-on-substrate filter,” Proc. IEEE Conf. on Microelectromechanical Sys., 176 (2008). These piezoelectric microresonators exhibit maximum Qs of a few thousand, limited by anchor losses, material damping in the metal electrodes used to transduce the devices and, especially in the case of lead zirconate titanate (PZT) resonators, by material damping in the piezoelectric layer.
The piezoelectric coupling coefficient kt2 determines the degree of coupling between the electrical and mechanical domains. Both high kt2 and high Q are desired to enable the widest range of filters to be designed (i.e., wide and narrow bandwidths with steep filter skirts). Aluminum nitride (AIN) and zinc oxide (ZnO) have been the materials of choice for piezoelectric microelectromechanical systems (MEMS) resonators because of their high coupling coefficients and quality factors; i.e., their kt2Q products. See H. Chandrahalim et al. “Influence of silicon on quality factor, motional impedance and tuning range of PZT-transduced resonators,” Solid-State Sensors, Actuators and Microsystems Workshop, 360 (2008). Dielectric losses at frequencies in excess of 1 GHz have been shown to limit the performance of ZnO based resonator designs. However, at frequencies >1 GHz, AIN is an excellent insulator. Ceramics, such as PZT, exhibit slightly higher coupling coefficients; however, high material damping limits their application, particularly at high frequencies. The highest reported fQ products for PZT resonators are 4×1011 with a kt2 of 9%. This is a factor of nearly 20 lower in performance compared to AIN with fQ=1013 and kt2=6.5%. See W. Pan et al. Here, f is the resonant frequency and Q is the quality factor of the acoustic resonance (i.e., the quality factor in a resonator is the ratio of the energy stored to lost energy per cycle or, equivalently, the resonator's bandwidth relative to its center resonant frequency, f/Δf).
Despite significant progress in the MEMS community, the highest Q devices available today are relatively large overtone bulk acoustic wave (BAW) devices. See G. R. Kline et al., “Overmoded high-Q resonators for microwave oscillators,” IEEE Freq. Cntl. Sym., 718 (1993). These thickness-mode BAW resonators comprise a piezoelectric resonator in which a vertical acoustic wave is generated within the piezolayer itself. The commercial community has refined thickness-mode resonators and evolved several variants, including solidly mounted resonators (SMRs), which include Bragg mirrors, and film-bulk acoustic resonators (FBARs). The resonant frequency is determined primarily by the thickness of the piezolayer, and by the electrodes and any additional mechanical layers. Resonators with resonant frequencies in the GHz range can have plate thicknesses of less than a few microns. The resonant frequency of these devices is very sensitive to the film thickness, which in today's best atomic layer deposition systems is still limited to the range of 0.1%, translating to 1-nm uniformity in a 1-μm thick film. Numerical, analytical, and experimental data place the sensitivity of fundamental thickness mode device frequencies on the order of 1 MHz/nm of film thickness. Therefore, the resonant frequency and bandwidth of thickness-mode resonators are difficult to control.
Therefore, a need remains for a resonator that can achieve a high Q factor with a narrow bandwidth in the GHz range, can maintain a low impedance level for low loss and RF power transmission, and can be lithographically defined using MEMS and CMOS compatible fabrication processes.